This invention is related to fingerprint pattern recognition, and more particularly, to enhanced fingerprint pattern sensing circuit and method.
Capacitive sensors to detect a fingerprint pattern are currently well known. There are a number of patents which describe capacitive sensors for determining the pattern of ridges and valleys in a person""s fingerprint. U.S. Pat. No. 4,353,056 describes a capacitive fingerprint sensor which has a sensing surface for receiving a fingerprint and sensing the ridges and valleys of the skin using an array of small capacitors. U.S. Pat. No. 5,325,442 also describes a fingerprint sensing and recognition circuit using a plurality of capacitors arranged in rows and columns.
Some capacitive sensors operate using two or more plates in silicon with a negative feedback as shown in FIGS. 1 and 2, taken from U.S. Pat. No. 5,973,623. In the multiple plate system, the finger of the user changes the capacitance that exists between the two plates in silicon. A negative feedback circuit is used in the detection of whether a ridge or a valley is present in the fingerprint.
Some capacitive sensors operate using a single capacitor plate in silicon per sensor cell. This type of sensor is shown in U.S. Pat. No. 5,325,442. FIGS. 3 and 4 are prior art. Figures from this patent illustrate a single capacitive plate 14 and a switching device 16 to access the sensor cell. The finger of the user provides the other plate of the capacitor. The value of the capacitor is then sensed to determine the presence of a ridge or a valley.
One of the problems with the prior art capacitive fingerprint sensors is their level of sensitivity to distinguish a ridge from a valley. The difficulty of obtaining a good fingerprint pattern through capacitive sensing can vary from individual to individual. With some persons, depending upon their skin characteristics the pattern can be much more difficult to obtain than from other persons. It is therefore preferred to have an enhanced method of capacitive sensing which provides more uniform sensitivity from one individual to another.
According to principles of the present invention, a capacitive fingerprint sensing device is provided which places a variable voltage on the body of the individual whose fingerprint pattern is being sensed. The finger is thus performing the dual function of providing the variable charge transfer during input while also providing the variable capacitance to be sensed. The effect of performing both functions effectively multiplies the measurable effects of having a ridge or a valley at each sensor cell. The sensing is significantly enhanced in recognizing and distinguishing the difference between a ridge and a valley.
The electrical connection to the person""s body can be made by any number of acceptable techniques. According to one preferred embodiment, an electrical contact is provided to the person""s finger while it is placed on the fingerprint sensor pad. A logic control circuit connected to the electrical conductor places a step voltage on the electrical conductor according to a preset timing sequence in conjunction with the timing of the sensing. In this embodiment, there is a direct connection between the variable input voltage and the finger of the user for which the fingerprint is being sensed.
In a further embodiment, the voltage change is placed on the person""s body by a capacitive transfer. A large plate capacitor in the silicon provides one plate to transfer charge to the finger to provide the change in voltage while the sensing occurs. Alternatively, the voltage change can be coupled to the body of the user either via another finger or some other appendage. For example, the connector can be in contact with the other hand of the person, their arm or any other acceptable portion of the body since all that is required is to place a change in charge on the person""s body in order to obtain the enhanced capacitive sensing capability.
The invention can be used with sensor cells of the type having one capacitive plate in silicon and the finger providing the other plate, or with the type using multiple capacitor plates in silicon and the finger being a conductor that modifies their relative capacitance.
In one embodiment, the fingerprint or sensor circuit includes a negative feedback amplifier having the two plates of a feedback capacitor in silicon with a field therebetween. The field between the two plates is varied by the user""s finger, whether ridge or valley. The effect of having a valley is that the sense capacitance will be greater than in the case of having a ridge. When a ridge is present, the feedback capacitance is reduced, as compared to a valley. In other words, the further the skin is from the plates of the capacitor, the less effect it has on reducing the effective capacitance between them and the sensed capacitance will be higher with a valley than with a ridge. On the other hand, the input capacitance will decrease with the greater distance of the skin from the sensor, such as is present at a valley. Since it is a ratio of the input capacitance to the sense (feedback) capacitance which determines the sensitivity of the circuit, a circuit which causes them to change in opposite directions results in a greatly multiplied effect of increasing the sensitivity of the measurement. Therefore, significant enhancement is provided by using the body as both the input capacitor and the variable plate to a capacitor whose value is being sensed.